In the present paper, the natural photooxidation degradation behavior and mechanism of PP containing two α-nucleating agents (TM-1 and TM-3) and a β-nucleating agent (TMB-5) were studied by Fourier transform infrared spectroscopy (FTIR) and infrared microscopy (IM) so that helpful instructions would be obtained for the PP material design and applications. The results showed that the nucleating agents had significant impact on the photooxidation stability of PP. The existence of them accelerated the photooxidation reaction of PP to different extents with the rank of PP/TMB-5＞PP/TM-3＞PP/TM-1＞PP. However, the photooxidation procedure and the corresponding oxidative products did not change. The oxidation profiles in PP and nucleated PP bars after exposure had similar trends. The carbonyl index curves had a maximum at the surface and declined gradually with distance from the exposed surface, then decreased to nearly zero at the depth of about 250 μm, which was named as the surface oxidation layer (SOL). The SOL thickness, which was dependent on the transmission of the ultraviolet light, the oxygen diffusion and mostly the “skin” layer formed during the injection moulding, did not change with exposure time. The development of oxidation along the depth coincided with the initiation and propagation of the surface cracks. Before fragments dropped from the seriously oxidized surface, the photooxidation degradation of PP was restricted mainly in the surface oxidation layer.